Too much radiation exposure is harmful. The degree of radiation
injury depends on the amount of radiation received and the time involved. In
general, the higher the amount, the greater the severity of early effects (occurring
within a few weeks) and the greater the possibility of late effects such as cancer.

The BEIR V (Biological Effects of Ionizing
Radiation) Committee of the National Research Council
estimates that among 100,000 people exposed to a one-time dose of 10 rem (10,000 mrem) and
followed over their life span, about 790 more would die of cancer than the estimated
20,000 cancer deaths that would be expected among a non-exposed group of the same size.
NOTE: 10 rem = 100 millisieverts (100 mSv).